Hydrostatic drive system

ABSTRACT

The invention relates to a hydrostatic drive system having a motor-driven hydraulic pump ( 1 ) that can be connected to at least one hydraulic drive unit ( 7 ) by means of a first ( 19 ) and a second working line ( 21 ) forming a hydraulic circuit, said drive unit being connected to a gear set ( 1, 3 ), having a first hydraulic accumulator ( 53 ) for accumulating pressure energy that can be connected to one of the working lines ( 19, 21 ), and a second hydraulic accumulator ( 55 ) that can be connected to the other working line ( 19, 21 ), and having a valve device (V 1 , V 2 ) by which the segment ( 23, 25 ) of each working line ( 19  or  21 ) running to the drive unit ( 7 ) can be separated in order to separate an accumulator part ( 33 ) from the part of the circuit comprising the hydraulic pump ( 13 ), said accumulator part comprising the hydraulic accumulators ( 53, 55 ) and the at least one drive unit.

The invention relates to a hydrostatic drive system having a motor-drivable hydraulic pump that can be connected to at least one hydraulic drive unit by means of a first and a second working line forming a hydraulic circuit, wherein one hydraulic accumulator, for recovery of stored pressure energy, can be connected to one of the working lines and a second hydraulic accumulator can be connected to the other working line.

Hydrostatic drives of this type, as are shown, for example, in document WO 2007/079935 A1, are often used in commercial vehicles like buses or mobile machinery. These drive systems offer the possibility of storing a part of the kinetic energy as pressure energy during braking processes and then recovering it, in order to support acceleration processes. In this respect, it is prior art for at least one drive unit to be set up as a hydraulic motor in the form of an adjustable motor-pump unit so that the hydraulic motor delivers hydraulic fluid in the overrun mode of the drive system. The delivery volume is supplied as a charging volume to the high pressure accumulator, with both a braking action in the drive unit and also a storage of pressure energy occurring in the high pressure accumulator. This energy can be recovered by the high pressure accumulator being connected to the working line that is assigned to the delivery side in the drive state for acceleration processes. In the respective working cycle for acceleration and braking, the second hydraulic accumulator—the low pressure accumulator—is connected to one of the working lines to equalize the volumes which have been removed or supplied to the respective other working line.

In light of the prior art, the object of the invention is to provide a drive system of the type under consideration that is characterized not only by an especially simple structure of the hydraulic circuit, but at the same time by an operating behavior which is improved relative to the prior art.

According to the invention, this object is achieved by a hydrostatic drive system which has the features of claim 1 in its entirety.

Accordingly, an essential particularity of the invention consists in that there is a valve device by means of which the segment of each working line which runs to at least one drive unit can be separated in terms of operation. By separating these segments of the two working lines, the hydraulic circuit is divided into two component circuits, specifically into a part of the circuit which contains the hydraulic pump, and an accumulator part which contains at least one drive unit and the hydraulic accumulator. In the normal drive state, that is, without acceleration or braking processes, there is no interruption of the working lines to the drive unit so that the drive unit is supplied from the hydraulic pump via the working lines. If an acceleration or braking process is to take place, the connections to the drive device are separated by the valve device. This means that the accumulator part is separated from the hydraulic circuit and for acceleration and braking processes therefore only the portion of lines and hydraulic components that belongs to the accumulator part is involved. This yields not only the advantage of a good response behavior as a result of the stiffness of the line system which encompasses only the accumulator part, thereby avoiding hysteresis phenomena, but the arrangement is also characterized by low flow losses due to the reduced line volumes through which the participating volumes are flowing. In contrast, in the aforementioned known solution in which there is no separation of an accumulator part which is active only during acceleration and braking, it is provided that in all operating states, volumes of hydraulic fluid flow actively through the entire circuit, as a result of which corresponding shifting losses and a correspondingly inert response behavior result.

In especially advantageous exemplary embodiments, there are two separate hydraulic drive units which are dynamically connected to one another in terms of operation, and by means of the valve device the segment of each working line which runs between the drive units can be separated. In acceleration and braking processes, therefore, after separation of the accumulator part in the other component circuit which, as a supply part, contains the hydraulic pump, a hydraulic drive unit also remains so that in acceleration processes both the drive unit which belongs to the accumulator part is supported by the energy which has been recovered from the high pressure accumulator, and also at the same time, the drive unit which belongs to the supply part is supplied by the hydraulic pump with drive energy. In effect, an addition of output from the output which originates from the recovered pressure energy and the energy delivered by the hydraulic pump to the drive unit which belongs to the supply circuit is thus produced.

The drive units can be mechanically coupled directly to one another, where the design is simple, for example, in traveling mechanisms; this is not critical, however. An operating dynamic connection could alternatively take place via transmission and/or clutch devices or simply such that one or more gear sets of each drive unit roll on a common raceway or roadway, that is, the drive units are not dynamically connected to one another propulsively, but only by jointly rolling on a common raceway.

In preferred exemplary embodiments, the valve device, by means of which the accumulator part can be separated from the hydraulic circuit in each working line, has a directional switching valve which is preloaded into the open state and which can be controlled by electromagnetic actuation into the blocked state.

With respect to the accumulator part, the arrangement can be made such that the accumulator part has a second valve device which is connected to the first hydraulic accumulator which is used as a high pressure accumulator for storage of pressure energy, and has a third valve device which is connected to the second hydraulic accumulator which is used as a low pressure accumulator. These second and third valve devices make it possible for the accumulator part, when an acceleration or braking process is not taking place, to remain inactive as it were, by the two hydraulic accumulators being separated from the circuit by means of the second and third valve devices, which are in the blocked state when separation of the accumulator part does not take place.

Here the arrangement is made such that the second and the third valve devices of the accumulator part both have one acceleration valve each, which can be controlled into the open state for an acceleration process in order to connect the high pressure accumulator to the first working line, which causes the drive pressure supply of the drive unit which belongs to the accumulator part and to connect the low pressure accumulator to the other, second working line for recovery of stored pressure energy. Furthermore, the second and the third valve devices of the accumulator part can both have one brake valve each which can be controlled into the open state for a braking process in order to connect the second working line, i.e., the working line which belongs to the delivery side of the drive unit in the overrun mode, to the high pressure accumulator and to connect the other, first working line to the low pressure accumulator for storage of pressure energy.

Preferably, both acceleration valves and also brake valves are each formed by directional valves which can be controlled into the open state by electromagnetic actuation and which are preloaded into the blocked state. Without electrical triggering, therefore the accumulator part, including the hydraulic accumulator, the safety components which are conventionally assigned to the hydraulic accumulators, and including the second and the third valve devices, is hydraulically separated from the rest of the circuit, which is active in the normal operating state.

The invention is detailed below using an exemplary embodiment shown in the drawings. The single FIGURE symbolically shows the hydraulic circuit of the example of the drive system according to the invention which is to be described here.

The invention is described using the example of a traveling mechanism which is intended, for example, for use in buses, with two drive units 5 and 7 being connected to one gear set 1 and 3 each. The first drive unit 5 is a fixed displacement motor which can be operated in two directions of rotation according to two volumetric flow directions. The second drive unit 7 is formed by an adjustable motor pump, likewise for two volumetric flow directions, according to the two directions of rotation. In this exemplary embodiment, the first drive unit 5 and second drive unit 7 are directly coupled mechanically via a connecting shaft 9.

For the energy supply of the system, a primary drive is provided by an internal combustion engine, in this exemplary embodiment in the form of a diesel engine 11 which drives a hydraulic pump 13 in the form of an adjustable pump which for the same direction of rotation can deliver in two volumetric flow directions. A feed pump 15 in the form of a fixed displacement pump can be driven jointly with the hydraulic pump 13 and, as discussed below, enables resupply of the system with hydraulic fluid from a tank 17 in order to compensate for leak losses of the system. To form a hydraulic circuit, a first working line 19 and a second working line 21 are connected to the two ports of the hydraulic pump 13; of the two lines, the first working line 19 is connected to a port A1 of the first drive unit 5, and the second working line 21 is connected to the other port B1 of the first drive unit 5. The working line 19 continues via a line segment 23 to the port A2 of the second drive unit 7, while the second working line 21 continues via a line segment 25 to the second port B2 of the second drive unit 7. In each of these line segments 23 and 25, there is a 2/2 directional valve V1 and V2. The latter are used as switching valves which in the open state clear the assigned line segment 23 and 25 or separate it in the blocked state. The directional valves V1 and V2 are mechanically preloaded into the open state and can be switched to the blocked state by electrical actuation of their actuating magnet 27.

An accumulator part, which is designated as a whole as 33 and which completes the entire hydraulic circuit, is connected via connecting lines 29 and 31 to the line segments 23 and 25 of the working lines 19 and 21.

The aforementioned feed pump 15, whose delivery side is protected by a pressure limiting valve 35 toward the tank side, is connected on the delivery side via nonreturn valves 37 and 39 to the working lines 19 and 21 in order to ensure the filled state of the system. With respect to the system pressure which prevails in operation, pressure limiting valves 41 and 43 are connected between the working lines 19 such that a limitation of the maximum pressure difference between the working lines 19 and 21 takes place. To limit the system pressure in the accumulator connecting lines 29 and 31 which are connected to the line segments 23 and 25, there are further pressure limiting valves 45 and 47 each toward the tank side. Moreover, a 3/2 directional valve 49, which can be opened via control pressure from the working line 19 or the working line 21 by pressure actuation, enables discharge to the tank side via a pressure limiting valve 51 which limits the discharge pressure. There is a pressure-voltage converter 54 for obtaining a signal for the pressure in the working line 19 on the line segment 23 of the working line 19 and thus on the associated accumulator connecting line 29.

The accumulator part 33 has a high pressure accumulator 53 and a low pressure accumulator 55 to which a respective accumulator safety block 57 is connected upstream and which are made in the known manner which is conventional in these accumulator arrangements so that a more detailed description of the safety blocks 57 is unnecessary. A valve device 63 and 65 is connected to the input 59 and 61 of each safety block 57, each valve device 63 and 65 containing two 2/2 directional valves at a time, specifically for the valve device 63 of the high pressure accumulator 53 one directional valve V4.3 and one directional valve V4.4, while the valve device 65 has one directional valve V4.1 and one directional valve V4.2. All of these directional valves are mechanically preloaded into the blocked state and can be electrically controlled into the open state by triggering their actuating magnet 67.

The FIGURE shows an operating state in which all electromagnetically controllable directional valves are de-energized. This state corresponds to the normal operating state of the system, without acceleration or braking. The directional valves V1 and V2 which are located in the line segments 23 and 25 of the working lines 19 and 21 are in the open state, while all directional valves which belong to the valve devices 63 and 65 of the accumulator part 33 are in the blocked state. The accumulator part 33 is therefore separated from the remaining circuit on the accumulator connecting lines 29 and 31. In the normal operating state, therefore the output of the drive units 5 and 7 is determined solely according to the delivery capacity of the hydraulic pump 13, where setting to a delivery capacity of zero can correspond to the pertinent vehicle at standstill.

In order to move the system into the initial operating state after the standstill of the vehicle, in which an acceleration process can take place with concomitant action of the accumulator part 33 for starting, there is a charging valve V3 in the form of a 4/2 directional valve whose first input 69 is connected to the first working line 19 and whose second input 71 is connected to the second working line 21. Outputs 75 and 73 are connected via a first charging line 79 and a second charging line 81 to the input 61 on the high pressure accumulator 53 and the input 59 on the low pressure accumulator 55. Both accumulators 53, 55 can therefore be charged to the desired pressure level when the actuating magnet 83 of the charging valve V3 which is preloaded into the closed state is actuated, the pressure-signal converters 85 and 87 delivering a display of the charging pressures.

When an acceleration process is to take place, for example, for starting off from a stop, or for accelerating while driving, the actuating magnets 27 of the directional valves V1 and V2 are triggered in order to block these valves. In this way, the line segment 23 of the working line 19 and the line segment 25 of the working line 21 are separated. The accumulator part 33 to which the second drive unit 7 belongs is separated from the hydraulic circuit, therefore from a remaining part, specifically the supply part which extends from the hydraulic pump 13 to the first drive unit 5. While in the illustrated exemplary embodiment in which there are two drive units 5 and 7, at this point the hydraulic pump 13 continues to remain hydraulically connected to the first drive unit 5, the activated accumulator part 33 is available for the desired acceleration process, with the energy which has been recovered from the high pressure accumulator 53 being supplied to the second drive unit 7. For this purpose, in the valve devices 63 and 65, those directional valves which are used as acceleration valves are controlled into the open state by energizing the magnets 67. In forward operation, this is the valve V4.4 in the valve device 63, and for the valve device 65, it is the valve V4.2. In this way, via the accumulator connecting line 29, the high pressure accumulator 53 is connected to the port A2 of the second drive unit 7, and, via the accumulator connecting line 31, the low pressure accumulator 55 is connected to the other port B2 of the drive unit 7. Thus the pressure energy of the accumulator 53 acts via the pressure-side input of the drive unit 7, while the volumetric flow delivered on the low-pressure side from the drive unit 7 is supplied to the low pressure accumulator 55 via the accumulator connecting line 31.

If, in contrast hereto, a braking process is to take place, where in turn the directional valves V1 and V2 are closed and the accumulator part 33 therefore is separated from the supply circuit on the segments 23 and 25, the directional valves V4.3 and V4.1 which are used as braking valves and which are contained in the valve devices 63 and 65 of the accumulator part are controlled into the open state. Thus, the high pressure accumulator 53 is now connected to the port B2 of the drive unit 7, whose other port A2 is connected to the low pressure accumulator 55 so that the pressure energy stored in the high pressure accumulator 53 is recovered as deceleration energy which is active on the second drive unit 7.

The operating mode of the system is described above based on forward operation or driving forward. Since the hydraulic pump 13 is an adjustable pump so that two volumetric flow directions are possible, the system can be easily operated in reverse. Of the directional valves located in the valve devices 63 and 65 of the accumulator part 33, the directional valves V4.3 and V4.1 in reverse now act as acceleration valves, while the directional valves V4.4 and V4.2 now act as brake valves.

While there are two drive units 5 and 7 in the illustrated embodiment, only one drive unit could be used which is active both in the normal drive state and also in acceleration and braking processes. In other words, in the description according to this FIGURE, the drive unit 5 could be omitted. Compared to this embodiment, however, the example described here is advantageous to the extent that both in the normal drive state, that is, with the valves V1 and V2 open and the accumulator part 33 inactive, the two drive units 5 and 7 are active and that also with the valves V1 and V2 closed, the first drive unit 5 in the supply part of the circuit remains active and is activated accordingly by the hydraulic pump 13. Since the second drive unit 7 is an adjustable motor pump with two opposite volumetric flow directions, operation of the hydrostatic drive system is ensured even if the valve modules V4.1 and V4.3 or V4.2 and V4.4 were to be dispensed with. 

1. A hydrostatic drive system having a motor-drivable hydraulic pump (13) that can be connected to at least one hydraulic drive unit (7) by means of a first (19) and a second working line (21) forming a hydraulic circuit, said drive unit being connected to a gear set (1, 3), having a first hydraulic accumulator (53) for storage of pressure energy, which can be connected to one of the working lines (19, 21), and a second hydraulic accumulator (55) which can be connected to the respective other working line (19, 21), and having a valve device (V1, V2) by which the segment (23, 25) of each working line (19 or 21) which runs to the drive unit (7) can be separated in order to separate an accumulator part (33) from the part of the circuit comprising the hydraulic pump (13), said accumulator part comprising the hydraulic accumulators (53, 55) and the at least one drive unit.
 2. The hydrostatic drive system according to claim 1, characterized in that there are two separate hydraulic drive units (5 and 7) which are dynamically connected to one another in terms of operation and that by means of the valve device (V1, V2) the segment (23, 25) of each working line (19, 21) that runs between the drive units (5, 7) can be separated.
 3. The hydrostatic drive system according to claim 2, characterized in that the drive units (5, 7) are mechanically coupled to one another.
 4. The hydrostatic drive system according to claim 1, characterized in that the valve device in each working line has a directional valve (V1, V2) which is preloaded into the open state and which can be controlled by electromagnetic actuation into the blocked state.
 5. The hydrostatic drive system according to claim 1, characterized in that the accumulator part (33) has a second valve device (63) which is connected to the first hydraulic accumulator, which is used as high pressure accumulator (53) for storage of pressure energy, and has a third valve device (65), which is connected to the second hydraulic accumulator (55), which is used as low pressure accumulator.
 6. The hydrostatic drive system according to claim 5, characterized in that the second and the third valve devices (63, 65) of the accumulator part (33) both have one acceleration valve (V4.4 and V4.2) each, which can be controlled into the open state for an acceleration process in order for recovery of stored pressure energy to connect the high pressure accumulator (53) to the first working line (19), which causes the drive pressure supply of the drive unit (7) which belongs to the accumulator part (33) and to connect the low pressure accumulator (55) to the other, second working line (21).
 7. The hydrostatic drive system according to claim 5, characterized in that the second and the third valve devices (63, 65) of the accumulator part (33) both have one brake valve (V4.3 and V4.1) each which can be controlled into the open state for a braking process in order to connect the second working line (21) to the high pressure accumulator (53) and the other, first working line (19) to the low pressure accumulator (55) for storage of pressure energy.
 8. The hydrostatic drive system according to claim 6, characterized in that acceleration valves and brake valves are each formed by directional valves (V4.1, V4.2, V4.3, and V4.4) which can be controlled into the open state by electromagnetic actuation and which are preloaded into the blocked state.
 9. The hydrostatic drive system according to claim 1, characterized in that there is a recharging valve in the form of a 4/2 directional valve (V3) which is preloaded into the blocked state and which can be controlled into the open state by electromagnetic actuation, in which it connects the high pressure side of the hydraulic pump (13) to the hydraulic accumulator which is used as the high pressure accumulator (53) and the low pressure side of the hydraulic pump (13) to the low pressure accumulator (55). 